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Let's talk about if
statements in Python.
if
statementsIf you'd like to run some code only if a certain condition is met, you can use an if
statement.
Here we have a program called language.py
:
value = input("What programming language are you learning? ")
if value == "Python":
print("Cool! This program was written in Python.")
In this program, we're prompting a user to enter a value, and then we're printing out a response only if the value that they entered is Python
:
$ python language.py
What programming language are you learning? Python
Cool! This program was written in Python.
If they enter anything else, we don't do anything at all:
$ python language.py
What programming language are you learning? JavaScript
We're using an if
statement to do this.
The if
statement has a condition, and if that condition is true, the block of code just after that condition is run.
else
with if
in PythonWhat if we wanted to print out a different response when our condition isn't met?
For that, we could use an else
statement.
An else
statement also comes after an if
:
value = input("What programming language are you learning? ")
if value == "Python":
print("Cool! This program was written in Python.")
else:
print(f"Neat. I haven't tried {value} yet.")
When the condition we're checking is true, we run the block of code right after the if
statement (the first print
line).
$ python language.py
What programming language are you learning? Python
Cool! This program was written in Python.
Otherwise, we run the block of code just after the else
statement (the second print
line).
$ python language.py
What programming language are you learning? JavaScript
Neat. I haven't tried JavaScript yet.
Our program now always performs an action, but the action will be different depending on the value the user provided:
elif
What if we had a variety of different conditions that we wanted to check one after the other, until one of them was true?
We could use elif
statements for that.
This light.py
program prints out a color of light based on a randomly generated wavelength:
from random import randint
wavelength = randint(390, 780)
print(f"{wavelength}nm wavelength")
if wavelength < 390:
print("Not visible")
elif wavelength < 455:
print("Violet")
elif wavelength < 492:
print("Blue")
elif wavelength < 577:
print("Green")
elif wavelength < 597:
print("Yellow")
elif wavelength < 622:
print("Orange")
elif wavelength < 780:
print("Red")
else:
print("Not visible")
This program works thanks to Python's if
, elif
, and else
statements.
These conditions in that if
-elif
chain cascade.
Meaning, Python looks through the conditions one-by-one, until it finds a condition that's true, it runs the block of code for that condition, and then it stops, jumping to the end of the if
statement chain.
Both else
statements and elif
statements are attached to the end of an if
statement.
An if
statement can have any number of elif
statements after it (including none at all) and it can optionally have a single else
statement at the end.
if
statementsIf you'd like to run some code only when a certain condition is met, you can use an if
statement in Python.
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